Chemical-mechanical planarization ("CMP") processes remove material from the surface of a semiconductor wafer in the production of integrated circuits. FIG. 1 schematically illustrates a CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a planarizing liquid 44 on the polishing pad 40. The polishing pad 40 may be a conventional polishing pad made from a continuous phase matrix material (e.g., polyurethane), or it may be a new generation fixed-abrasive polishing pad made from abrasive particles fixedly dispersed in a suspension medium. The planarizing liquid 44 may be a conventional CMP slurry with abrasive particles and chemicals that remove material from the wafer, or the planarizing liquid 44 may be a planarizing solution without abrasive particles.
The CMP machine 10 also may have an under-pad 25 attached to an upper surface 22 of the platen 20 and the lower surface of the polishing pad 40. A drive assembly 26 rotates the platen 20 (as indicated by arrow A), or it reciprocates the platen 20 back and forth (as indicated by arrow B). Since the polishing pad 40 is attached to the under-pad 25, the polishing pad 40 moves with the platen 20.
The wafer carrier 30 has a lower surface 32 to which a wafer 12 may be attached, or the wafer 12 may be attached to a resilient pad 34 positioned between the wafer 12 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier to impart axial and/or rotational motion (as indicated by arrows C and D, respectively).
To planarize the wafer 12 with the CMP machine 10, the wafer carrier 30 presses the wafer 12 face-downward against the polishing pad 40. While the face of the wafer 12 presses against the polishing pad 40, at least one of the platen 20 or the wafer carrier 30 moves relative to the other to move the wafer 12 across the planarizing surface 42. As the face of the wafer 12 moves across the planarizing surface 42, material is continuously removed from the face of the wafer 12.
In the competitive semiconductor industry, it is desirable to consistently stop CMP processing of a run of wafers at a desired endpoint and to produce a uniform, planar surface on each wafer. Accurately stopping CMP processing at a desired endpoint is important to maintaining a high throughput of planarized wafers because the thickness of the planarized layer on the wafer must be within an acceptable range. It will be appreciated that if the thickness of the planarized layer is not within its acceptable range, the wafer must be re-planarized until it reaches a desired endpoint. Additionally, it is important to accurately produce a uniform, planar surface on each wafer to enable precise circuit and device patterns to be formed with photolithography techniques. The critical dimensions of many photo-patterns must be focused within a tolerance of approximately 0.1 .mu.m. Focusing photo-patterns to such small tolerance, however, is difficult when the planarized surface of the wafer is not uniformly planar. Therefore, two primary objectives of CMP processing are stopping planarization at a desired endpoint and producing a highly uniform, planar surface on each wafer.
CMP processing involves many operating parameters that affect the planarity of the surface on the wafer and the ability to stop CMP processing at the desired endpoint. The rate at which material is removed from the surface of the wafer (the "polishing rate") often varies across the face of a wafer and from one wafer to another. The most common parameters that affect the polishing rate of a wafer are: (1) the relative velocity gradient between the wafer and the polishing pad across the face of the wafer; (2) the distribution of slurry across the surface of the wafer; (3) the composition of materials across the wafer; (4) the topography of the wafer; (5) the parallelism between the face of the wafer and the surface of the polishing pad; (6) the temperature gradient across the face of the wafer; and (7) the condition of the planarizing surface of the polishing pad. The polishing rate may vary across the face of the wafer because any one of the operating parameters may change during planarization. Moreover, the polishing rate may vary from one wafer to another because it is difficult to identify and correct changes in specific operating parameters. Thus, it is difficult to consistently stop CMP processing at a desired endpoint on a wafer by estimating the time-to-polish using the polishing rate of previous wafers.
One desirable technique of endpointing CMP processing is the stop-on-feature ("SOF") wafer design. A typical SOF wafer has a polish-stop layer at a desired endpoint on the wafer and a cover layer over the polish-stop layer. The polish-stop layer is made from a material that has a low polishing rate relative to the polishing rate of the cover layer; high regions of the cover layer are accordingly removed faster than lower, exposed portions of the polish-stop layer. The objective of the polish-stop layer, therefore, is to prevent or slow further polishing beyond the high point of the polish-stop layer.
Although SOF wafers are a promising technique to endpoint CMP processing, they may not consistently stop CMP processing at a desired endpoint in some applications. In conventional SOF wafers, the polishing rate of the polish-stop layer is less than that of the cover layer because the polish-stop layer is harder than the cover layer (to inhibit mechanical removal) and/or the polish-stop layer has a lower etch rate in the planarizing solution than the cover layer (to inhibit chemical removal). Although the polishing rate of the polish-stop layer is lower than the cover layer, material is still generally removed from the exposed portions of the polish-stop layer. The removal of material from the polish-stop layer is a particularly acute problem for thin polish-stop layers because exposed portions of thin polish-stop layers may be completely removed from the wafer, thereby destroying the structure protected by the polish-stop layer. The removal of material from the polish-stop layer also reduces the uniformity of the planarized surface. Therefore, conventional SOF wafers may not consistently stop CMP processing at a desired endpoint with conventional CMP processing methods.